Fermented milk such as yogurt is produced by adding a starter to raw material milk (yogurt mix) into which raw milk, skimmed milk powder, whey protein, or the like are mixed and fermenting the yogurt mix. As the starter, used is lactic acid bacterium such as Lactobacillus bulgaricus, Streptococcus thermophilus, or the like.
After the production, the yogurt mix is cooled to a temperature of 0 to 10 degrees and shipped in cold storage. Since the lactic acid bacterium is alive in the yogurt in cold storage, the yogurt is gradually fermented in the course of distribution and during the period of storage at home or the like and the acidity thereof increases. As a result, the flavor of the yogurt is changed even within the freshness date.
It is well known that some kinds of lactic acid bacterium produces an antibacterial protein or peptide called a bacteriocin, As shown in the following Patent Documents 1 to 3, it is possible to improve the preservative quality of foods by using the bacteriocin-producing lactic acid bacterium.
In the invention of Patent Document 1, bacteriocin-producing Lactococcus lactis is used as a starter for yogurt. Lactococcus lactis produces a bacteriocin as the yogurt mix is fermented. Since the bacteriocin produced by Lactococcus lactis suppresses an increase in the acidity of yogurt, it is possible to improve the preservative quality of yogurt.
In the invention of Patent Document 2, Bifidobacterium and Lactococcus lactis are cocultured by using a liquid culture medium of which the main ingredients are milk and milk constituents. By adding the culture solution after the coculture to foods (bread, Udon noodles (Japanese noodles), or the like) as a food preservative, it is possible to improve the preservative quality of the foods and give good flavor to the foods.
Patent Document 3 shows a flavor improving agent obtained by culturing Lactococcus lactis with a whey medium to which yeast extract or the like is added and removing the Lactococcus lactis from the whey medium after the culture. By using this flavor improving agent, it is possible to get rid of the fishiness of the fish (the fishy odor) and give good flavor (taste) to foods.    [Patent Document 1] Japanese Patent Application Laid Open Gazette No. 4-211360    [Patent Document 2] Japanese Patent Application Laid Open Gazette No. 8-187071    [Patent Document 3] Japanese Patent Application Laid Open Gazette No. 2004-283109
As discussed above, the invention of Patent Document 1 uses the bacteriocin-producing Lactococcus lactis as a starter. For this reason, it is difficult to suppress an increase in the acidity of the yogurt for which Lactococcus lactis is not used as a starter.
For the purpose of suppressing an increase in the acidity of the yogurt in the course of distribution and during the period of storage, the food preservative used in Patent Document 2 or the flavor improving agent used in Patent Document 3 may be added the yogurt mix as an additive. In a case where the additive is added to the yogurt mix at a certain rate, however, there is a possibility that the yeast extract which is a material of the additive may damage the original flavor of the yogurt. Therefore, in the case where the culture of the bacteriocin-producing lactic acid bacterium is added to the yogurt mix, it is desirable that the amount of culture to be added should be made as small as possible.